1. Field of the Invention
The present invention relates to a knocking detector for an internal combustion engine.
2. Description of the Prior Art
When strong knocking continues in an internal combustion engine, the engines durability is adversely affected. It is known, however, that slight knocking at relatively low engine speeds brings about best engine output and fuel economy. There is a close relationship between the occurrence of knocking and the ignition timing; the further the ignition timing is advanced, the more knocking occurs.
Conventionally, a system has been proposed wherein the knocking state of the engine is sensed to adjust the ignition timing to maintain a slight knocking state, thereby improving fuel economy and engine output.
It is known that when knocking occurs, the engine vibrates, thereby resulting in an increase of the amplitude of the vibration component with a particular frequency. The above conventional system utilizes this fact; it includes a sensor which senses the vibration of the engine body, selects a band of particular frequency vibration components, removes a component corresponding to the background noise level to determine the knocking intensity from the remaining portion, and retards the spark timing when knocking stronger than a predetermined level occurs to limit the knocking to an appropriate magnitude.
Actually, it is very difficult to determine the level of the background noise. For example, where the output signal of a vibration sensor is rectified and thus smoothed to obtain a signal corresponding to the background noise level, this signal contains knocking components and so does not reflect exactly the background noise level. Thus, when knocking occurs, the knocking strength is likely to appear to be lower than its actual value.
It is an object of the present invention to provide a knocking detector which determines the knocking strength with high accuracy.